In monopulse antennae, several radiation patterns are used simultaneously and their shapes have a direct influence on the overall performance of the radar system including such antennae. Monopulse techniques use in fact simultaneously several patterns coming from the same antenna; in so-called amplitude operation, a distinction is made on the one hand between a pattern with even symmetry or `sum` pattern serving as a reference and, on the other hand, patterns with odd symmetry or `difference` patterns giving elevation and azimuthal angular-deviation-measurement signals with respect to the axis of the antenna.
In so-called `phase` operation, the angular-deviation-measurement signals are obtained by comparing the phase between two patterns having the same amplitude function. It should moreover be noted that it is possible to pass from one operating mode to the other by means of a coupler system, so that in the rest of this description only the case of amplitude operation will be considered.
In these different operating modes, the patterns used are represented mathematically by orthogonal functions, which involves decoupling the corresponding channels.
On the other hand, the different radiating characteristics of these patterns, which have a direct influence on the performance of the system, are not a priori independent but are interlinked by restricting relationships depending on the structure of the antenna. These characteristics are the gain and the level of the side lobes in the sum channel and the difference channels, the slope in the vicinity of the axis and the level of the main lobes in the difference channels.
For a given antenna structure, the problem raised is tantamount to trying to find an optimization between the factors which have been already mentioned, with consideration given to their relative ranking imposed by the functions of the system considered. It may be deduced therefrom that any structure possesses an optimization field, but conventional antennae structures have shown their limits in the case of monopulse techniques. In fact it has proved impossible in conventional monopulse antennae to control independently the sum and difference patterns for properly controlling the shape of the illumination function for the primary source, which is particularly important in the construction of low-noise antennae for radio astronomy and spatial telecommunication. The conventional monopulse technique has also shown its limits in the application to telecommunication antennae for tropospheric transmission in which the diversity between the sum and difference channels is utilized.
To remedy these limitations, multimode sources have been developed for use in antennae of corresponding type.
A multimode source or moder is capable, by virtue of its peculiar structure, of generating direct propagating modes with controllable phases and amplitudes allowing a desired illumination in its aperture to be obtained.
Generally, a moder is a structure formed of waveguides comprising discontinuities at which higher modes are generated.
A study of such moders may be found, among others, in French Pat. No. 1,290,275 and our commonly owned prior U.S. Pat. No. 4,241,353 which relates to a combined multimode structure formed by combining an E-plane moder with an H-plane moder, as shown in present FIG. 1 which is representative of the prior art.
Such a structure allows independent control of the sum and difference patterns to be obtained in the E and H planes. However, such control does not take place simultaneously in these planes but successively.
The structure of FIG. 1 is formed by two flat moders ME.sub.1, ME.sub.2 placed side by side and separated by a common vertical partition. Each of these moders is energized by two pairs of guides 1, 10 and 2, 20 which receive the basic mode and which open into a guide 3, 30 of a length L.sub.1 between planes P.sub.0 and P.sub.1. Plane P.sub.0 is what is called the plane of discontinuity at which there are formed higher modes, propagating or evanescent, length L.sub.1 and the dimensions of guides 3, 30 being such that only the desired modes, in this case for example the odd modes H.sub.11 and E.sub.11 and the even modes H.sub.12 and E.sub.12, are propagated as far as the opening of the E-plane moder thus formed, i.e. the plane P.sub.1, the basic mode being the mode H.sub.10.
Starting at plane P.sub.1 are H-plane moders designed to provide the desired distribution functions in the horizontal plane without distorting the distribution functions established in the vertical plane by the E-plane moders ME.sub.1 and ME.sub.2. Metal plates 4, 40, 5, 50, 6, 60 disposed horizontally in a guide 8, 80 of length L.sub.2, forming a continuation of guides 3 and 30 beyond plane P.sub.1, define four pairs of adjacent horizontal flat guides which adjoin each other at their small sides and are energized in accordance with the distribution functions defined by the moders ME.sub.1 and ME.sub.2. The horizontal plates extend beyond plane P.sub.2 in a guide 7 having the shape of a horn of length L.sub.3.
The assembly located between planes P.sub.1 and P.sub.3 forms a stack of H-plane moders, plane P.sub.2 being the plane of discontinuity where higher modes are formed. The aperture of the combined structure, which is located in a plane P.sub.3, radiates according to an overall illumination function, which is a product of the partial illumination functions obtained in the vertical plane and in the horizontal plane.
Multimode sources or feeds of the kind just described are used in radar antennae, more particularly in tracking radar, but they have the drawback of requiring considerable space in the longitudinal direction, which is troublesome for the construction of certain antennae in which an improved performance, principally regarding the passband, causes an increase in inertia impairing the operation of the servo-mechanisms.
In our above-identified prior patent we have disclosed a multimode feed free from the aforementioned disadvantages, comprising a structure for a combined E-plane and H-plane moder by which, besides a reduction in the dimensions of the source, an increase of the passband in the H-plane is realized.
FIG. 2 gives a view of such a moder in which the increase of the passband is obtained by providing the aperture 16 of a horizontally flared horn 13 with vertical metal bars or strips 14, 15 and 140, 150 disposed parallel to the electric field of the emitted wave.